Structures comprised of fiber reinforced plastic

ABSTRACT

Unitary hollow structures, for example radomes, comprised of fiber reinforced plastic are produced with a large percentage of the fibers being randomly oriented in directions essentially parallel to the wall surfaces of the structures and with at least one surface having grooves to reduce microwave reflection. The structures are produced by packing a layer of fiber filled polymeric material in powder form around a mandrel which may have a pattern of grooves or ridges in its outer surface. The mandrel and packed powder are subjected to isostatic pressing to properly orient the fibers and achieve a density increase and powder cohesion. The pressed structure is sintered and the outer surface subsequently machined to a finished contour.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 342,057, filedJan. 25, 1982, now U.S. Pat. No. 4,623,505, which is acontinuation-in-part of U.S. patent application Ser. No. 263,191 filedMay 13, 1981, now U.S. Pat. No. 4,615,859. The application is also adivisional of U.S. Pat. No. 4,623,505.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention is directed to the fabrication of unitarystructures of complex shape, such as missile radomes, from fiberreinforced plastic material. More particularly, the present inventionrelates to hollow structures, such as radomes, which have improvedlongitudinal strength and, in the case of a radome, improvedelectromagnetic energy transmission characteristics.

(2) Description of the Prior Art

While not limited thereto in its utility, the present invention isparticularly well suited for use in the manufacture of radomes.Accordingly, the discussion below will be primarily related to radomes.

Ceramic radomes are typically used for missiles intended to operate atspeeds of Mach 4 or higher. These ceramic radomes have been found to beat best marginal in performance due to fragility, susceptability tothermal shock, high thermal conductivity and high rates of rain impactdamage. A definite need for a workable alternative to ceramic radomesexisted for many years.

Radomes made from polymeric materials have been suggested as a possiblealternative to ceramic radomes Polytetrafluoroethylene, hereinafterPTFE, is one such polymeric material which might be suitable for radomeapplications. However, "neat" or simple filled PTFE does not possess therequisite characteristics, uniformity of erosion and ablation forexample, for use in the demanding environment of a missile radome. Testshave shown that fiber reinforced PTFE, i.e., a PTFE composite with thefibers having a high aspect ratio, would have those characteristicsdictated by radome and similar usage.

Prior to the invention disclosed in application Ser. No. 149,952, nowU.S. Pat. No. 4,364,884, it had been a practical impossibility tofabricate a radome from a PTFE-fiber composite. The production of asolid block of PTFE composite of sufficient size to permit machining aradome therefrom is not feasible due to the virtual impossibility ofheating such a large block through the crystalline melt point andsubsequently cooling through the recrystallization point with enoughuniformity of temperature to avoid fissures and damage from thermalstress. Furthermore, even if the temperature gradient and thermal stressproblems could be avoided, an extremely long heating and cooling cycle(perhaps on the order of several weeks) would be required, and that longcycle time would result in thermal degradation. Other approaches, suchas flowing a sheet of PTFE composite material to form a radome shape orlaminating a series of rings or discs cut from such sheet material allinvolve substantial technical or cost problems which precluded the useof such material and techniques.

My U.S. Pat. No. 4,364,884 discloses a novel radome structure comprisedof a fiber reinforced plastic material wherein the fibers are, to a highdegree, randomly oriented in planes which are perpendicular to the axisof the radome. This novel fiber reinforced plastic radome ismanufactured by sintering together preformed segments of the radomewhile maintaining axial pressure upon the segments. The preformedsegments are formed by cold pressing a powdered PTFE-fiber compositematerial into rings or discs, the cold pressing step causing the fibersto become oriented randomly in planes perpendicular to the axes of thediscs. These discs are machined to form a series of preforms of desiredsize and shape. The preforms are stacked within a mold cavity andsubjected to heat and axial pressure. The resulting unitary sinteredstructure is machined to form the final desired product.

The final unitary product produced in accordance with the teachings ofthe above-mentioned U.S. patent overcomes many of the disadvantages ofthe prior art. It has excellent resistance to ablation and rain erosionand is not as fragile as previous ceramic radomes. Also, a fiberreinforced radome produced in accordance with the teachings of U.S. Pat.No. 4,364,884 is economical to produce when compared to the cost ofmachining a radome from a large block of PTFE-fiber composite.

However, a radome produced in accordance with the invention of U.S. Pat.No. 4,364,884 possesses. characteristics which limit its usage. Forexample, since the fibers are oriented in planes perpendicular to theradome axis, the longitudinal tensile strength of the structure iscomparatively low. Accordingly, a supporting liner is needed in somecases. The liner will typically be comprised of a glass fiber-epoxystructure or a polyimide-glass fiber honeycomb structure. The bonding ofa supporting liner within a previously formed radome may result in theradome fracturing or there may be incomplete bonding between the radomeand the supporting liner. The problems associated with bonding a linerwithin a radome are due in part to the radome having a much higherdegree of thermal expansion in the axial direction than does thesupporting liner. When fracturing and/or incomplete bonding occurs itwill happen during the processing step when heat is applied to cure theadhesive used to bond the liner to the radome. Either voids will formbetween the liner and the radome due to the radome expansion or theradome will fracture due to tension as it contracts on cooling if thereis adequate bonding to the liner. It has also been observed that whenexposed to low temperatures the bonded radome and liner assemblyexperiences axial stresses due to the differences in thermal expansion.These stresses result in tension between the radome and liner which canlead to fissure formation.

SUMMARY OF THE PRESENT INVENTION

The present invention overcomes the above-discussed disadvantages andother deficiencies of the prior art by providing a novel unitarystructure of complex shape and comprised of a fiber reinforced compositematerial, such as a radome, and a method of manufacture thereof.

Thus, in accordance with the present invention a product comprised offiber-reinforced polymeric material is produced wherein the fibers areto a high degree randomly oriented in planes which are perpendicular tolines which are normal to the inner surface of the radome. Longitudinalstrength is greatly improved and lower thermal expansion co-efficient inthe circumferential and longitudinal directions is obtained because ofthis fiber orientation. It is to be noted that resistance to ablationand rain erosion are not as great in the case of a radome produced inaccordance with the present invention as in the case of the radomedisclosed in U.S. Pat. No. 4,364,884. However, a radome in accordancewith the present invention has sufficient resistance to rain erosion andablation to be acceptable for many applications. The method of thepresent invention includes uniformly packing a thoroughly blendedmixture of a polymeric material and reinforcing fibers in particulateform around a mandrel which is supported in a mold cavity. The mandrelhas a surface contour which is commensurate with the desired contour ofthe interior of the structure to be produced. The layer of compositematerial formed about the mandrel is subjected to sufficient externalpressure for a sufficient period of time to compact the powder to almostits ultimate desired density. In order to assure that a large percentageof the fibers become oriented in planes which are perpendicular to linesnormal to the surface of the mandrel, the pressure should be appliedequally over the entire exposed surface of the layer of compositematerial in a direction normal to the mandrel surface. The preferredmethod this pressure is a known isostatic pressing for applying thispressure is a known isostatic pressing technique. The mandrel andcomposite material are enclosed in a sealed flexible bag to preventpenetration of the pressing fluid into the composite material. It isfurther preferable to evacuate any air from within the bag and polymericmaterial powder in order to prevent fissures from developing in theformed layer when the pressure being applied is released.

After the composite material layer has been compacted by the appliedpressure it is subjected to a sufficiently high temperature to fuse orsinter the polymeric material. In the case of PTFE, this temperatureshould range between 350° C. to 400° C. Furthermore, in order to reducethe possibility of cracking or fissure formation within the radome, orother structure, this heating is carried out in an inert atmosphere. Ifthe powder layer is heated while still positioned around the mandrel itis essential to maintain the temperature differential across the layerof composite material between the mandrel and the surrounding atmospherewithin a narrow range. This is especially crucial when the temperatureis being raised through the crystalline melting temperature of PTFE andwhen it is being lowered through the recrystallization temperature ofPTFE. If the temperature difference between the mandrel and surroundingatmosphere becomes too great, the radome may crack or fissure.

After the mandrel, if still present, and the cured PTFE layer are cooledto room temperature, the PTFE layer is finished by machining it to thedesired dimensions of the product, for example a radome, being formed.

It has been determined that the reflection of microwave energy from aradome produced in accordance with the present invention may be reducedby providing either or both of the inner and outer surfaces of a radomecomprised of a polymeric material-fiber composite with grooves. For amissile radome, if only one surface is to be grooved, it isaerodynamically better to provide the grooves on the inner surface. Inaccordance with a further aspect of the present invention, inner surfacegrooves are formed by compacting the composite material about a mandrelwhich is provided with a selected groove pattern. This pattern maycomprise either a series of longitudinal grooves or one or more helicalgrooves. After the powder has been compacted and sintered about themandrel the finished radome structure is removed. If the groove is ofhelical shape removal is effected by a twisting action. Grooves may beprovided in the exterior surface after the radome is finished bysuitable known machining procedures and, if provided, are preferrablylongitudinal.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be better understood and its numerous objectsand advantages will become apparent to those skilled in the art byreference to the accompanying drawing wherein like reference numeralsrefer to like elements in the several Figures, and wherein:

FIG. 1A and 1B depicts, in cross-sectional side elevation, two mandrelswhich may be employed in the novel manufacturing process of the presentinvention, polymeric composites being indicated schematically on themandrels;

FIG. 2 is a flow diagram of the novel process of the present invention;

FIG. 3 shows a side elevational view, partially in section, of afinished radome in accordance with the invention;

FIGS. 4A and 4B are cross-sectional views illustrating the step offorming a layer of composite material around a mandrel in practicing themethod of FIG. 2;

FIG. 5 is a cross-sectional view of a mandrel and composite materiallayer in position within an elastic bag for compacting by an isostaticpressing technique;

FIG. 6 is a side elevational view, partially in section, of a finishedradome with a supporting liner bonded therein;

FIG. 7 is a cross-sectional view of a mandrel with a helically shapedsurface groove;

FIG. 8 is a side elevational view, partially in section, of a finishedradome having grooves provided in both its exterior and interiorsurfaces;

FIG. 9 is a cross-sectional view of the radome shown in FIG. 8 takealong line 9--9;

FIG. 10 is an end view of another mandrel for use in the practice of thepresent invention; and

FIG. 11 is a side elevation view, partly in section, of a finishedradome produced using the mandrel of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention consists of a novel unitary structures comprisedof fiber-reinforced plastic material and methods of manufacture thereof.It is to be noted that while a radome and the manufacture thereof willbe discussed and illustrated, the invention is not limited to such use.

With reference now to the drawing, and particularly to FIGS. 1A, 1B and2, the first step in the practice of the present invention involvespacking a mixture comprising reinforcing fibers and a polymericmaterial, preferrably polytetraflouroethylene (hereinafter PTFE), toform a layer 12 around mandrel 10. It is to be noted that the layer 12has been shown as a double broken line to indicate that the fiberreinforced plastic is initially in the form of a powder which issubsequently compacted to reduce its thickness and increase its density.Mandrel 10 is preferably comprised of aluminum and has a surface contour16 which corresponds to the desired contour of the interior surface ofthe radome. Mandrel 10 is prepared by any conventional machiningtechnique and may be reused for processing numerous radomes of thedesired shape. Preferably, mandrel 10 is provided with undercut 14. Thefunction of undercut 14 will be discussed below.

Layer 12 is a thoroughly blended mixture of PTFE in powder form andreinforcing fiber. This blended mixture is prepared by a dry processwhich provides an intimate blending of the PTFE particles with theindividual fibers. Also, the PTFE powder is sifted through a screenbefore blending to insure against lumps. Two examples of compositematerials, i.e., thoroughly blended polymeric material-fiber mixtures,capable of use in the practice of the present invention are "RT/duroid"types 5650M and 5870M available from Rogers Corporation, Rogers, Conn.and comprising by weight:

    ______________________________________                                        "RT/duroid" type       5650 M  5870 M                                         "Teflon" 7A (polytetrafluoroethylene,                                                                75%     85%                                            available from E. I. duPont)                                                  Ceramic fibers         25%      0%                                            (aluminum silicate fibers of random                                           size and having an average diameter                                           of about 1 μm and an average length                                        exceeding 100 μm)                                                          Glass Microfibers      0       15%                                            (available from the John-Manville Corp.                                       and having an average diameter of                                             about 0.2 μm and an length                                                 exceeding 30 μm)                                                           ______________________________________                                    

The final compounded powder, i.e., the PTFE-fiber mixture, has apreferred bulk density of about 0.25 grams/cubic centimeter.

The reinforcing fibers useful in the practice of the present inventionmay be comprised of a ceramic material, glass microfibers or othersimilar material. The fibers, which are inorganic, will typically rangein diameter from 0.05 to 10 micrometers and will preferably have anaspect ratio of at least 30. The final fiber content of the mixtureshould range between 5% and 40% by weight.

While the above discussion has been limited to the use of only PTFE,other fluoropolymers may be added to the PTFE powder for the purpose ofmodifying the processing requirements or for obtaining certain desirablecharacteristics. Typically, such additives will possess lower meltingtemperatures, lower melt viscosity, better ability to wet fiber orfiller surfaces, and better ability to close voids. Other types of PTFEresins which may be used are Teflon 7C or other commercially availablegranular or coagulated dispersion types of PTFE. Finally, meltprocessible fluoropolymers, such as Dupont's "Teflon FEP" or "TeflonPFA" may be added to serve as an aid to coalescence during the sinteringstep.

It is further possible to prepare the PTFE-fiber composite as an aqueousslurry. If the aqueous slurry process is employed a PTFE dispersion isadded along with a flocculating agent to a mixture of water and fiber.This slurry is then dewatered, by vacuum, against a mesh fabric coveredform, preferably a perforated shell shaped similarly to the mandrel 10.The resulting low density "pulp form" shape has, after drying, an insidediameter resembling the form. A PTFE dispersion useful in the practiceof the present invention is "Fluorn" AD704, produced by ICI, America.

With reference to FIGS. 4A and 4B, a preferred method of forming thePTFE-fiber composite layer of the present invention about mandrel 10 isdepicted. A mold 28 is provided with a cavity 30 that is larger than theexterior contour of the radome to be formed. The cavity configuration isdesigned to allow for the bulk factor of the mixture so as to insurethat the layer 12, in its final configuration is sufficiently oversizedto permit machining the outer contour by cutting away a minimal amountof trim. An elastic bag 32, having approximately the same shape ascavity 30, is positioned within cavity 30. The open end of bag 32 isstretched over the mold 28 and sealed against the exterior surfacethereof. The space between the bag 32 and the cavity 30 is evacuated bya high volume pump (not shown) which is connected to cavity 30 throughpassages provided in mold 28 (also not shown). This conforms bag 32 tothe surface of cavity 30.

Mold 28 is positioned upon a base plate 34. Three posts 38, only two ofwhich are shown, extend upwardly from plate 34. Posts 38 are arrangedtriangularly and are provided with threaded ends 40. A Y-shaped supportplate 42 is supported on posts 38 by passing threaded ends 40 of theposts through apertures 44 in plate 42. Plate 42 is secured at a desiredheight by nuts 46 as shown.

A mandrel support shaft comprising a pair of interconnected rods 48 and50 extends from Y-shaped support plate 42. Rod 48 is provided withexternal threads at both ends while rod 50 is provided with only oneexternally threaded end. The second end of rod 50 has an internallythreaded blind hole which engages a first end of rod 48. The other endof rod 48 passes through an aperture 54 in plate 42 and is held to plate42 by a pair of nuts 52.

The externally threaded end of rod 50 engages mandrel 10. Mandrel 10 islowered into cavity 30 of mold 28 until the desired spacing between thewall of cavity 30 and mandrel 10 is achieved. This spacing should besufficient to allow the appropriate amount of PTFE composite powder 36to be delivered into bag 32. An elastomeric plug 56, preferably in twosections, is positioned around the mandrel support shaft. Plug 56 isprovided with a hole 58 which, while allowing passage of rods 48 and 50,provides a tight enough fit to seal a vacuum. Plug 56 is also providedwith a cavity 62, which receives a self-sealing rubber stopper 64, andan evacuation port 66 which extends from the bottom of cavity 62.

The PTFE composite powder 36 is seived into the space between mandrel 10and bag 32. Caution must be taken while loading the powder 36 to insureeven distribution of the powder within bag 32 and light tamping and/orvibration may be employed. After bag 32 is fully filled, it is closed.This is accomplished by sliding elastomeric disc plug 56 down shafts 48and 50 until it contacts mandrel 10. The bag 32 is then taped to theplug 56, preferably by plastic pressure-sensitive tape 60, as shown inFIG. 4B. This prevents liquid intrusion into bag 32 during isostaticpressing step 20.

It has been found that by evacuating air from within the bag 32 andpowder 36, fissures are prevented during the pressure release stage ofisostatic pressing step 20. In order to permit evacuation of bag 32, afabric strip 68 is positioned between the two sections of plug 56 in thevacinity of port 66 before bag 32 is sealed to the plug. The fabricstrip defines a gas flow path between the sections of the plug andfunctions as a filter which prevents the evacuation of powder. A largebore hypodermic needle 70 is then pierced through stopper 64 into port66. The air is drawn out of bag 32 and powder 36 by attaching needle 70to a vacuum pump (not shown) through tubing 72. This evaluation stepwill typically consist of pumping down the sealed and powder filled bagfor at least one hour. After the air has been withdrawn, the needle 70is removed and, since stopper 64 is self-sealing, the interior of bag 32will remain air free. Next, as shown in FIG. 5, rod 48 is disengagedfrom rod 50 and is replaced by machine screw 74 and stopper 76. Thisensures proper sealing. A second elastomeric bag 78 is then placed overplug 56 and taped to bag 32 by tape 80.

Once the air has been evacuated from bag 32 and powder 36 and the bag 78sealed to plug 56 and bag 32, isostatic pressing step 20 is commenced.This involves removing the mandrel 10 with bag 32 and composite powderlayer 36 from mold 28 as a unit and placing this unitary assembly in acold isostatic press which consists of a high pressure vessel (notshown) filled with water or other suitable liquid that will not degradebags 32 and 78. The pressure of the liquid is raised slowly to themaximum desired value, preferably 30,000 psi, over a time span of aboutan hour. The maximum pressure is held for about 5 minutes. The pressureis then slowly reduced at a constant rate to 14.7 psi over a time spanof 45 to 60 minutes. The controlled release of pressure is typicallyachieved by a high pressure needle value. Caution must be taken not torelease the pressure too rapidly. If the pressure is released toorapidly, the compacted powder layer may fracture. While the above arethe preferred pressures and times for the isostatic pressing of a PTFEcomposite layer, the maximum pressure may range from 5000 psi to 100,000psi and be reached within 30 to 60 minutes. The maximum pressure shouldbe held between 1 to 10 minutes. Furthermore, it is also possible toreduce the pressure from the maximum to atmospheric pressure within 5 to60 minutes. As noted above, mandrel 10 is provided with undercut 14.This insures that layer 12 of compacted PTFE composite is locked andretained upon mandrel 10 after the completion of pressing step 20.

After completion of isostatic pressing step 20 the powder has beencompacted into a layer 12 (FIGS. 1 and 6) which is very nearly at theultimate desired density and which has a major percentage of fibersoriented as desired. The fibers in layer 12, before the isostaticpressing step 20, are randomly oriented substantially equally in alldirections. The pressure applied during step 20 is in a direction normalto the surface of mandrel 10. This causes a large percentage of thefibers within the powder being compacted to become randomly oriented inplanes which are perpendicular to lines which are normal to the nearestsurface of mandrel 10. This may be contrasted to the technique of U.S.Pat. No. 4,364,884 wherein the principal fiber orientation is in planeswhich are perpendicular to the radome axis.

After completion of pressing step 20, the mandrel 10 and compressedlayer 12 are subjected to a sintering step 22. This involves removingthe mandrel 10 and layer 12 from the elastomeric bag 32 and subjectinglayer 12 to a temperature ranging between 350° C. to 400° C., with thepreferred temperature being 380° C. This heating is carried out byplacing the mandrel 10 with layer 12 in a forced circulation oven whichis provided with an inert atmosphere, preferably nitrogen. The sinteringtemperature is reached within 3 to 30 hours and held between 1 to 8hours. The mandrel and layer 12 are then cooled to room temperature.Caution must be taken during cooling and heating the mandrel 10 andlayer 12 to insure that a significant temperature differential is notestablished across the layer 12. This is especially critical as thetemperature passes through the crystalline melting temperature of thePTFE and also as the temperature is lowered through therecrystallization temperature of the PTFE. If the temperature differencebetween the mandrel 10 and the exterior surface of the PTFE compositelayer becomes too great the radome may crack. While sintering with layer12 still positioned upon mandrel 10 is the preferred procedure, it isalso possible to remove layer 12 from mandrel 10 prior to sintering step22. This is accomplished by either machining the layer 12 to remove thelocking tabs which engage undercut 14 or employing a mandrel which doesnot have the undercut. The sintering temperature and times remain thesame for both methods.

With sintering step 22 completed, layer 12 is finished by machining itto the desired dimensions of the radome. If layer 12 remains uponmandrel 10 during sintering step 22, the mandrel 10 may be used as asupport fixture for the concentric finishing of the outside contour oflayer 12. The completed radome is obtained by removing layer 12 frommandrel 10. This is accomplished, as noted above, by a machiningoperation to separate the material around undercut 14.

It has been found that an added advantage of retaining layer 12 uponmandrel 10 during sintering step 22 is that the final percentage offibers having the desired orientation is improved. This is a result oflayer 12 being locked to undercut 14. Normally, layer 12 would creep upas it shrinks during the heating. By being locked to undercut 14, layer12 must stretch as it shrinks in order to accommodate mandrel 10. Thiscauses further compression of layer 12 in a direction normal to the axisof mandrel 10.

It should be apparent from the above discussion that the preferredtechnique of retaining layer 12 upon mandrel 10 during the sinteringstep 22 reduces the machining requirements and improves the final radomeproduct.

With reference now to FIG. 3, a finished radome is indicated generallyat 26. It should be apparent that radome 26, which is comprised of acompacted and sintered layer 12 of PTFE composite, may be produced inany desired shape by using an appropriately designed mandrel 10.

By providing the surfaces of a radome with grooves, exterior or interioror both, microwave reflection may be reduced. This reduction ofreflection broadens the useable frequency range for the particularradome. It is believed that this reduction in reflection is a result ofa gradual transition from the dielectric constant of the air to thedielectric constant of the radome material which results from theuniformly uneven surface contour.

Referring jointly to FIGS. 8 and 9, a radome 26 is shown with internalgrooves 84 and external grooves 86. The internal grooves 84 are formedby molding the blended powder of PTFE and fibers around a mandrel 10(FIG. 7) which is provided with a circumferentially oriented groove 88in the form of a spiral or helix. In this manner the radome 26 can beremoved from the mandrel 10 after it is sintered by twisting. The resultis a radome 26 which has an inwardly spiraling groove 84, as seen bestin FIG. 9, on its inner surface.

The exterior surface of radome 26 is provided with longitudinal grooves86 during the final machining step 24. Preferably, the grooves 86 arelongitudinal and radiate outwardly from the tip 90 of radome 26, but anyconfiguration is possible.

Referring now to FIGS. 10 and 11, another embodiment of the presentinvention will be discussed. FIG. 10 shows a mandrel 92 which isprovided with longitudinal grooves 94 which forms longitudinal grooves96 in the interior surface of radome 26 of FIG. 11.

It is to be noted that when the mandrel is provided with an irregularsurface contour, for the purpose of forming grooves in the interiorsurface of the radome, the fiber orientation will be tangent to theaverage of the nearest mandrel surface. Thus, in the vicinity of thegrooves 96 there will be regions wherein the majority of the fibers willnot lie in planes which are perpendicular to lines normal to the mandrelsurface.

It is further to be noted that, in discussing fiber orientation herein,applicant is referring to mutually orthogonal X, Y and Z axes within thePTFE composite material, with the Z axis being normal to the interiorsurface of the radome. Employing this convention, the least number offibers are oriented in the Z direction because of the direction of theapplied pressure during the compacting step.

While preferred embodiments have been described and illustrated,modifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, the presentinvention has been described by way of illustration and not limitation.

What is claimed is:
 1. A unitary structure, said structure having anaxis and comprising:a tip portion, said tip portion being symmetricalwith respect to the axis of the unitary structure and being continuous;a tubular portion extending from and integral with said tip portion,said tubular portion being coaxial with said tip portion and terminatingat an annular surface having an exterior diameter greater than themaximum diameter of said tip portion; said tip and tubular portionshaving internal and external surfaces and being comprised of a fiberfilled polymeric composite, the majority of the fibers comprising saidcomposite being oriented in planes which are perpendicular to linesnormal to the average contour of the interior surface of the unitarystructure; and grooves being formed on at least one of the internal orexternal surfaces of said tip and tubular portions, said groovesreducing microwave reflection.
 2. The article of claim 1 wherein thefibers comprise from 5 to 40 parts by weight of the composite.
 3. Thearticle of claim 1 wherein:said polymeric composite comprises afluoropolymeric composite.
 4. The article of claim 1 wherein:saidunitary structure comprises a radome.
 5. The article of claim 4including:a supporting liner within said radome.
 6. The article of claim1 including:said grooves are formed in a spiral or helix on the interiorsurface of said tip and tubular portions.
 7. The article of claim 1including:a plurality of longitudinally extending grooves on theinterior surface of said tip and tubular portions.
 8. The article ofclaim 1 including:a plurality of longitudinally extending grooves on theexterior surface of said tip and tubular portions.